A computer tomographic device and a method for correcting the radiation hardening is known from DE 100 51 462 A1. The known device has an x-ray source and an x-ray detector that together rotate around an object to be examined. The projection images taken by the x-ray detector are applied to an evaluation unit that corrects the radiation hardening. To do this the evaluation unit performs a post-reconstructive correction procedure. As part of the post-reconstructive correction procedure, the evaluation unit first reconstructs approximate volumetric images from the object to be examined from the uncorrected projection images. The term volumetric image in this case, and in the following, means both three-dimensional volumetric views and two-dimensional section images. A reprojection is then performed, with only those pixels being used in the volumetric image whose the image value is above a specified threshold value and that are interpreted as materials to be distinguished from soft tissue. These materials can, for example, be bones or a contrast medium. The limitation to specific pixels enables the computing expense for the reprojection to be reduced.
With conventional computer tomography, a constant voltage is used for all projection directions, except for small fluctuations due to the generator for the tube voltage. The tube voltage is in this case preferably chosen so that the radiation dose received by the detector is adequate for all projection directions and object thicknesses. If the object to be examined is a patient, the patient under certain circumstances is exposed to a dose of radiation that is greater than would be necessary to take the particular projection image.
Devices and methods have therefore been developed to reduce as far as possible the radiation dose to which the patient is exposed. A device and a method of this kind are, for example, known from U.S. Pat. No. 6,222,907 B1. With the known device and known method, the parameters of the x-ray tube are controlled corresponding to the beam path through the object being examined.
The application areas for the known device and known method are radiography and fluoroscopy.
In recent times, the C-arch device for rotational angiography has been continuously improved. In particular, the mechanical stability of the C-arch has been increased, thus enabling approximate rotation about an isocenter. Together with the use of area detectors with an increased dynamic compared with x-ray image amplifiers, this enables a computer tomography volumetric reconstruction.